Image pickup device and image pickup system

ABSTRACT

There is provided an image pickup device, including a photoelectric conversion element converting light into charges, a transfer gate for transferring the converted charges to a floating node, a source follower transistor for outputting a signal based on a voltage of the floating node to a signal line, and a clip circuit clipping the signal line at a first voltage and a second voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup device and an imagepickup system.

2. Description of Related Art

In recent years, a higher-image quality inexpensive digital camera hasbecome popular owing to a progress in the performance of the imagepickup device. In particular, a CMOS sensor, which has an active elementin each pixel and peripheral circuitry on the chip, has remarkablyimproved its performance, and as a result, CCD sensors have partiallybeen replaced by CMOS sensors. The CMOS sensor has an active element forconverting electric charges into an optical signal output in each pixel.The threshold variation of each pixel, and kTC noise (thermal noise) ata time of resetting causes fixed pattern image noise and random imagenoise. For removing these types of noise, correlated double sampling(CDS) for reading only image signals by acquiring the difference betweena reset noise output after resetting and an output after a chargetransfer has been proposed.

In the following passage, a problem at the time of performingphotography using a CMOS sensor performing the CDS is described. When avery bright light source is photographed in a photographing region,strong light irradiates the electric charge conversion part of the CCDsensor. Consequently, the reset noise output varies owing to the light,and the dynamic range of the active circuit is suppressed. As a result,the image signal of the pixel irradiated by the strong light is reduced(hereinafter this phenomenon is referred to as an image signal reductionat the time of a large quantity of light). For example, when the sun isphotographed, the center part of the sun becomes a black point andbecomes an unnatural image. This problem is solvable when taking a stillimage by providing a mechanical shutter. However, in a movie, becausethe use of the mechanical shutter is disadvantageous for securing anadequate exposure time and a frame speed, a mechanical shutter is notoften used to solve this problem. Moreover, because an inexpensivecamera frequently omits the mechanical shutter, the problem also occurseven at the time of photographing a still image. In view of suchproblems, a method of suppressing the optical signal output reduction atthe time an image containing a large quantity of light is incident on animage pickup device has been proposed.

Japanese Patent Application Laid-Open No. 2000-287131 proposes a methodof detecting an output variation at a time of reading a reset noise towrite a predetermined value as a reset noise output when the outputvariation is judged to be caused by a large light quantity. According tothe proposal, at the time of reading an output after a charge transfer,an image signal output reduction prevention circuit is in a cutoff stateat the time of receipt of the large light quantity, and does notespecially affect an image.

However, the image region other than the pixel which is irradiated by astrong light with a large light quantity is sometimes influenced by thelight. FIG. 3 is an explanatory diagram of a CMOS sensor. Each ofreference numerals 301 to 303 denotes a unit pixel cell, and is arrangedin two dimensions. Reference numerals 304 and 305 denote constantcurrent sources provided to each column. The constant current sources304 and 305 constitute source follower amplifiers together with thesource follower transistors in the pixel cells 301 to 303. A common gatevoltage 307 is given to the constant current sources 304 and 305, and acommon power source wiring 306 is connected to the constant currentsources 304 and 305. A signal of each of the pixels 301 to 303 is readfrom output terminals 308 and 309 at each row.

When a strong light having an intensity equal to or more than asaturated output irradiates the pixel 302, the voltage of the outputterminal 308 falls, and it deviates from the working range the constantcurrent source 304. As a result, a predetermined current is led not toflow through the constant current source 304, and the current quantityflowing through the power source wiring 306 decreases. The constantcurrent sources 305 of the other columns are influenced by the variationof the current, and the voltages of the output terminals 309 are variedto influence the image.

The influence is described with reference to the schematic view of FIG.4 at the time of a window chart image pickup. A reference numeral 401denotes a dark output region or not saturated output region, and thedark output region 401 corresponds to the pixels 301 of FIG. 3. A lighthaving an intensity equal to or more than the saturated outputirradiates a region 402 corresponding to the pixel 302 of FIG. 3. Areference numeral 403 denotes regions that are irradiated by the samelight as that irradiating the regions 401, and the regions 403correspond to the regions 303 of FIG. 3. An image shaped in a stripe ina lateral direction in the regions 403 is formed under the influence ofthe saturated region 402.

SUMMARY OF THE INVENTION

An image pickup device according to the present invention comprises aplurality of pixels, each including a photoelectric conversion elementconverting light into charges, a transfer gate for transferring theconverted charges to a floating node, a first transistor for outputtinga signal based on a voltage of the floating node to a signal line, and asecond transistor for resetting the voltage of the floating node; aconstant current source supplying a current from drain to source of thefirst transistor, and a clipping circuit capable of limiting the signalline at a first voltage and a second voltage different from the firstvoltage.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an image pickup device according to afirst embodiment;

FIG. 2 is an explanatory diagram of drive pulses of the image pickupdevice according to the first embodiment;

FIG. 3 is an explanatory diagram of a CMOS sensor;

FIG. 4 a schematic view at the time of a window chart image pickup;

FIG. 5 is a block diagram showing an example of the configuration of astill video camera according to a third embodiment;

FIG. 6 is a block diagram showing an example of the configuration of avideo camera according to a fourth embodiment; and

FIG. 7 is an explanatory diagram showing a second embodiment.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a circuit diagram of an image pickup device according to afirst embodiment, and FIG. 2 shows the drive pulses thereof. Although acase where an electron is used as a charge and the transistors are Nchannel MOS transistors (NMOS's) is described here, it is possible toobtain the advantages of the present embodiment also in the case wherethe types of the transistors and the polarities of pulses are inverted.A reference numeral 101 denotes a unit pixel cell. The unit pixel cellsare repeatedly arranged in two dimensions, although the showing of themis omitted in the diagram. Hereinafter, a unit pixel is referred as apixel. In the present embodiment, the pixel cell 101 employs athree-transistor system, and is configured not to include any selectiontransistors in the pixel cell 101.

The pixel cell 101 is concretely composed of a photodiode 102, afloating capacity 103, a transfer gate 104, a source follower transistor107 and a reset transistor 105. The gate of the source followertransistor 107 is connected to a floating node 108, and the drainthereof is connected to a power source 109. The source of the resettransistor 105 is connected to the floating node 108, and the resettransistor 105 is controlled by the reset gate 106. The photodiode 102converts a received light into charges by photoelectric conversion, andstores the converted charges. The charges are also called photocharges.A plurality of pixel cells 101 is connected to a vertical signal line110.

The drain of the reset transistor 105 and the source of the sourcefollower transistor 107 are connected to the vertical signal line 110.The vertical signal line 110 is connected to a constant current source111. The constant current source 111 supplies a current from drain tosource of the source follower transistor 105. And a source followeroperates. The source follower transistor 107 outputs the voltage of thefloating node 108 to the vertical signal line 110. Then, the voltage ofthe vertical signal line 110 is read from the output terminal 112.

The output is held at a sample hold circuit S/H(N). The presentembodiment includes two sample hold circuits denoted by referencecharacters S/H(N) and S/H(S), respectively, in FIG. 1. In the samplehold circuit S/H(N), a reset signal (hereinafter referred to as an Nsignal) based on the voltage of the floating node 108 at the time ofresetting the floating node 108, is held. In the sample hold circuitS/H(S), a signal (hereinafter referred to as an S signal) based on thevoltage of the floating node 108 at the time of transferring the chargesof the photodiode 102 is held.

Herein, a period of reading and holding a signal based on a voltage ofthe floating node at the time of reset of the floating node is referredto as a reset signal reading period. And, a period of reading andholding a signal based on a voltage of the floating node at the time oftransferring the charge from the photoelectric conversion element to thefloating node is referred to as a photoelectric conversion signalreading period.

Hereupon, the S signal is a signal including the N signal and a signalbased on the charges transferred from a photodiode. Accordingly, asdescribed above, it becomes possible to acquire an image signal byoperating the difference between the S signal and the N signal.

A row selection is performed by controlling the voltage of the floatingnode 108.

To put it concretely, the row selection is controlled by a voltageVresL113 supplied from a transistor 114 and a voltage VresH115 suppliedfrom a transistor 116, both the transistors 114 and 116 being connectedto the vertical signal line 110. The voltage VresH115 is higher than thevoltage VresL113. These voltages VresH115 and VresL113 are hereinafterreferred to as reset voltages.

The driving is concretely described. The transistors 114 connected tothe vertical signal lines 110 are turned on. Then, the reset transistors105 of all of non-selected rows and a selected row are turned on at thesame time as the low reset voltage VresL113 is written in the verticalsignal lines 110. By this operation, the vertical signal lines 110 andthe floating nodes 108 of all of the pixels are reset at the lowervoltage VresL113.

Next, only the reset transistor 105 of a row to be desired to beselected, i.e. only the reset transistor 105 of a certain column, ismade to be in a state of being turned on, and the transistors 114 areturned off. After that, the transistor 116 of the column is turned on.By this operation, the high reset voltage VresH115 is written in thefloating node 108 of the pixel to be desired to be selected. That is,the voltage of the floating node 108 is made to be the reset voltageVresH115 by turning on the transistor 116 and the reset transistor 105.

Furthermore, the vertical signal line 110 is operated as a sourcefollower by turning off the transistor 116. Although a plurality ofsource follower transistors 107 is connected to the same vertical signalline 110 at this time, only the source follower of the highest voltage,i.e. the source follower at the selected row in which the higher resetvoltage VresH115 is written, becomes effective, and a signal dependingon the floating node voltage of the selected row is output to the outputterminal 112.

The driving is described with reference to the timing chart of FIG. 2.

A reference character PresL denotes a pulse supplied to the gates of thetransistors 114. A reference character PresH denotes a pulse supplied tothe gates of the transistors 116. A reference character Res(NON-SELECTED ROW) denotes a pulse supplied to the gates of the resettransistors 105 of non-selected rows, and a reference character Res(SELECTED ROW) denotes pulses supplied to the gates of the resettransistors 105 of a selected row. A pulse marked by the referencecharacter S/H(N) is a pulse at the time of performing the sample hold ofthe N signal, and a reference numeral Tx denotes a pulse supplied to thetransfer gate 104 in the pixel cell 101. A pulse marked by the referencecharacter S/H(S) is a pulse at the time of performing the sample hold ofthe S signal. A reference character Vclip will be described later.

First, the sample hold circuit S/H(N) (FIG. 1) performs the sample holdof the N signal of the selected row, in which the high reset voltageVresH115 is written, which is read by the method described above, at thetiming of the signal S/H(N). Next, the photocharges from the photodiode102 are transferred to the floating node 108 at the timing indicated bythe reference character Tx. After that, the sample hold circuit S/H(S)(FIG. 1) performs the sample hold of the voltage of the output terminal112, i.e. S signal, at the timing of the signal S/H(S). Then, an imagesignal according to an incident light is read by operating on thedifference between the S signal and the N signal, although the componentfor the operation is not shown in FIG. 1.

Hereupon, a clip circuit is provided to each of the vertical signallines 110. This operation of the clip circuit is to limit, within apredetermined range, the voltage of the signal line exceeding thepredetermined range. This voltage of the predetermined range may be avoltage level determined based on, for example an image signal, adynamic range or the like of the constant current source. The clipcircuit in the present embodiment includes a clip transistor 118, apower source 117 and switching means 119. A clip voltage Vclip (FIG. 2)is applied to the gate of the transistor 118. The clip circuit iscapable of switching the voltage of the signal line by means of the gatevoltage of the transistor 118. It is possible to make the power source117 output the same voltage as that of the power source 109 of thesource follower transistor 107 of the pixel cell 101. Moreover, it ispreferable to make the size of the clip transistor 118 be the same asthat of the source follower transistor 107 of the pixel cell 101.

In the present embodiment, the voltage of the clip voltage Vclip isgiven as a pulse shown in FIG. 2. By the switching means 119, a voltageVclipH is given at the time of reading the N signal, and a voltageVclipL is given at the time of reading the S signal. The clip transistor118 supplies a first voltage VclipH′ to the vertical signal line 110when the voltage given to the gate of the clip transistor 118 is thevoltage VclipH, and supplies a second voltage VclipL′ to the verticalsignal line 110 when the voltage given to the gate is the voltageVclipL.

That is, during a period of at least a part of the reset signal readingperiod, the voltage of the signal line is limited to the first voltageVclipH′. And, during a period of at least a part of the photoelectricconversion signal reading period, the voltage of the signal line islimited to the second voltage VclipL′.

Because the S signal is clipped by the voltage VclipL′ in case of thelight quantity equal to or more than the saturated output, the voltageof the vertical signal line 110 does not fall too much. At this time,the voltage VclipL′ is set to be higher than the voltage at which theconstant current source 111 is turned off. Consequently, because acurrent continues to flow in the constant current source 111, it ispossible to suppress the generation of a lateral stripe.

Moreover, according to the present embodiment, for example, even when avery bright subject such as the sun has been photographed, namely, evenwhen the voltage of the floating node 108 has been significantly changedin a period from a reset of the floating node 108 to a reading of areset noise, the voltage of the reset noise output does not fall to avoltage smaller than the voltage VclipH regulated by the voltage VclipH.As a result, VclipL′ may be determined based on the dynamic range or thelike of the image signal after CDS. Then, because the voltage VclipL′regulated by the voltage VclipL is output to the output terminal 112similarly at the time of the usual saturation at the time of the signaloutput after a charge transfer, the image signal Vsig is expressed bythe following formula.Vsig=|VclipL′−VclipH′|

As a result, a constant image signal output can be acquired at the timeof receipt of the large quantity of light.

As described above, the clip transistor 118 supplies the first voltageVclipH′ to the vertical signal line 110 when the sample hold of a Nsignal is performed by the sample hold circuit S/H(N), and supplies thesecond voltage VclipL′ to the vertical signal line 110 when the samplehold of a S signal is performed by the sample hold circuit S/H(S). Thefirst voltage VclipH′ and the second voltage VclipL′ are voltagesdifferent from each other. It is preferable that the first voltageVclipH′ is higher than the second voltage VclipL′.

By the above, it is possible to reduce the fall of the voltage of thesignal line at the time of outputting a N signal to a voltage smallerthan the voltage VclipH′, and to reduce the fall of the image signaloutput at the time of the large light quantity. Moreover, the influencesto the outputs of the other pixel cells can be suppressed by supplyingvoltage VclipL′ at the time of the signal.

Consequently, it becomes possible to make it difficult to generate theimage signal output decrease at the time of a large light quantity, andit also becomes possible to make it difficult to form an image of astripe at the pixels other than a saturated pixel at the time of receiptof the large light quantity.

Moreover, by the voltage relation between the first voltage VclipH′ andthe second voltage VclipL′, it becomes possible to make it difficult togenerate a decrease of the image signal output, which is sometimesgenerated at a pixel on which a strong light is irradiated even if thebrightness thereof is equal to or smaller than the level at which asaturated output is output, and it also becomes possible to make itdifficult to generate a stripe when a light having the light quantity isequal to or more than a saturated light quantity.

Moreover, in the present embodiment, the switching means 119 performsthe clipping by the two voltages from the transistor 118. For example,two groups, each consisting of the transistor 118 and the power source117 like the present embodiment, are provided. Hereupon, voltagesdifferent from each other are supplied from the two power sources, andthe switching means 119 switches on and off of the transistors. Thereby,clipping using different voltages can be performed. As described above,the clip circuit is not limited to the circuit configuration of thepresent embodiment.

Second Embodiment

Suitable bias conditions are described as a second embodiment. It isassumed that a voltage at which the constant current source 111 beginsnot to perform its normal operation owing to the excessive falling ofthe voltage of the output terminal 112 is denoted by a referencecharacter Vlimit, and that a range of capturing an image signal (thedifference between S signal and N signal), namely the saturation rangeof the capture of an analog/digital (A/D) converter 6 (FIG. 5) at asubsequent stage, is denoted by a reference character Vrange. The A/Dconverter 6 converts the voltage output through the output terminal 112from an analog signal into a digital signal.Vlimit>VclipL′Vsig=VclipL′−VclipH′|Vsig|>|Vrange|

That is, the difference Vsig between the first voltage VclipH′ and thesecond voltage VclipL′ is larger than the saturation range of the A/Dconverter.

Furthermore, the relations between light quantities and output voltagesare described with reference to a diagram showing the relationsschematically in FIG. 7. For simplification, it is supposed that thedirection indicated by the arrow of the ordinate axis indicating thevoltages is negative.

In FIG. 7, the S signal and the N signal increase as the light quantityincreases. The S signal and the N signal severally have an inclinationdifferent from each other. Then, the N signal is clipped at the firstvoltage VclipH′, and the S signal is clipped at the second voltageVclipL′. The situation is shown with the solid lines and the dottedlines, and it is supposed that the light quantities at the time ofstarting the clippings are denoted by reference characters a and b,respectively. And the voltage Vlimit is expressed by a dotted line.

Hereupon, the difference Vsig between the S signal and the N signal isan image signal. In FIG. 7, a reference character P denotes the imagesignal. The image signal should usually increase at a fixed rate withrespect to the quantity of incident light. However, the image signal Pdecreases between the light quantities a and b.

Accordingly, in the present embodiment, what is necessary is just tomake the difference Vsig, larger than the saturation range Vrange of theA/D converter 6. By means of setting the image pickup device to meetsuch a condition, an adverse effect on the image signal due to avariation of the voltages of the vertical signal lines caused by theplurality of the clip circuits can be avoided.

By setting the image pickup device to fulfill such a condition, it ispossible to reduce the generation of a stripe image, and to reduce thedecrease of the image signal at the time a large light quantity isincident on the image pickup device, and further, it is made possible toreduce the variation of the image signal and then to produce a highquality image signal. And, a single A/D converter may be arranged in theimage pickup device. And, also a plurality of the A/D converters may bearranged in the image pickup device. For example, the A/D converters maybe arranged per each of the vertical signal lines 110. In this case, thesignal reading speed can be improved. And, by means of converting theanalog signal from the pixel into a digital signal, loss in transmittingand an adverse effect due to the noise can be reduced.

Third Embodiment

An example of applying the image pickup device to an image pickup systemis shown below. With reference to FIG. 5, a case where the image pickupdevice is applied to a still video camera is described in detail. FIG. 5is a block diagram showing an example of the configuration of the stillvideo camera. The image pickup device of FIG. 1 is used as a solid stateimage pickup device 4.

In FIG. 5, a reference numeral 1 denotes a barrier commonly used as aprotector of a lens and a main switch. A reference numeral 2 denotes thelens focusing an optical image of a subject on the solid state imagepickup device 4. A reference numeral 3 denotes a diaphragm for varyingthe light quantity of the light passing through the lens 2. Thereference numeral 4 denotes the image pickup device for capturing thesubject focused by the lens 2 as an image signal. A reference numeral 5denotes an image signal processing circuit performing analog signalprocessing of an image pickup signal (image signal) output from theimage pickup device 4. The reference numeral 6 denotes the A/D converterperforming the analog-to-digital conversion of an image signal outputfrom the image signal processing circuit 5. A reference numeral 7denotes a signal processing unit performing various corrections of imagedata output from the A/D converter 6 or compressing data. A referencenumeral 8 denotes a timing generator outputting various timing signalsto the image pickup device 4, the image signal processing circuit 5, theA/D converter 6 and the signal processing unit 7. A reference numeral 9denotes an arithmetic-operation, entire-still-video-camera control unitfor performing various operations and controlling the entire still videocamera. A reference numeral 10 denotes a memory unit for temporarilystoring image data. A reference numeral 11 denotes an interface unit forperforming recording or reading from or to a recording medium 12. Thereference numeral 12 denotes the detachably attachable recording medium,such as a semiconductor memory, for performing recording or reading ofimage data. A reference numeral 13 denotes an interface unit forperforming communication with an external computer and the like. Theimage signal processing circuit 5 and the A/D converter 6 may be formedon the same semiconductor substrate as that on which the image pickupdevice 4 is formed, and may be formed by the same process step as thatfor producing the image pickup device 4.

Next, the operation of the still video camera at the time ofphotographing in the configuration described above is described. Whenthe barrier 1 is opened, the main power source is turned on, and thenthe power source of a control system is turned on, and further the powersource of image pickup system circuits such as the A/D converter 6 andthe like is turned on. Then, in order to control light exposure, thearithmetic-operation, entire-still-video-camera control unit 9 makes thediaphragm 3 open, and a signal output from the image pickup device 4 isconverted by the A/D converter 6 through the image signal processingcircuit 5. After that, the converted signal is input into the signalprocessing unit 7. The arithmetic-operation, entire-still-video-cameracontrol unit 9 performs the exposure operation based on the data. Thearithmetic-operation, entire-still-video-camera control unit 9determines the brightness based on the result of having performed thephotometry, and controls the diaphragm 3 according to the result.

Next, the arithmetic-operation, entire-still-video-camera control unit 9extracts high-frequency components based on a signal output from theimage pickup device 4, and performs an operation to determine thedistance to the subject. After that, the arithmetic-operation,entire-still-video-camera control unit 9 drives the lens 2 to judgewhether the lens 2 is in-focus or not. When the arithmetic-operation,entire-still-video-camera control unit 9 determines that the lens 2 isnot in-focus, the arithmetic-operation, entire-still-video-cameracontrol unit 9 again drives the lens 2 to perform an operation. Then,after the ascertainment of being in-focus, main exposure starts. Afterthe exposure has been completed, an image signal output from the imagepickup device 4 is subjected to the A/D conversion by the A/D converter6 through the image signal processing circuit 5, and passes through thesignal processing unit 7 to be written in the memory unit 10 by thearithmetic-operation, entire-still-video-camera control unit 9. Afterthat, the data stored in the memory unit 10 is recorded on thedetachably attachable recording medium 12 such as a semiconductor memoryor the like through the I/F unit controlling recording medium 11 by thecontrol of the arithmetic-operation, entire-still-video-camera controlunit 9. Moreover, the data may be directly input a computer or the likethrough the external I/F unit 13 to be subjected to the processing ofthe image.

Thus, according to the present embodiment, high quality still image canbe provided.

Fourth Embodiment

An example of applying the image pickup device to another image pickupsystem is shown below. With reference to FIG. 6, an embodiment of thecase where the image pickup device is applied to a video camera isdescribed in detail. The solid state image pickup device of FIG. 1 isused as an image pickup device 23.

A reference numeral 21 denotes a taking lens composed of a focus lens21A for performing focusing, a zoom lens 21B performing a zoomoperation, and a lens 21C for image formation. A reference numeral 22denotes a diaphragm and mechanical shutter. The reference numeral 23denotes the image pickup device performing the photoelectric conversionof a subject image formed on an image pickup surface to convert thesubject image into an electric image pickup signal. A reference numeral24 denotes a sample hold circuit (S/H circuit) which performs a samplehold operation on the image pickup signal output from the image pickupdevice 23 and amplifies the level of the sampled image pickup signal tooutput an image signal.

A reference numeral 25 denotes a process circuit which performspredetermined processing of an image signal output from the sample holdcircuit 24, such as gamma correction, color separation, blankingprocessing and the like, and outputs a luminance signal Y and a chromasignal C. The chroma signal C output from the process circuit 25 issubjected to the corrections of white balance and color balance by thecolor signal correction circuit 41, and is output as chrominancedifference signals R-Y and B-Y.

Moreover, the luminance signal Y output from the process circuit 25 andthe chrominance difference signals R-Y and B-Y output from the colorsignal correction circuit 41 are modulated by the encoder circuit (ENCcircuit) 44 to be output as a standard television signal. Then, thestandard television signal is supplied to an unillustrated videorecorder or an electronic view finder such as a monitor electric viewfinder (EVF).

Subsequently, a reference numeral 26 denotes an iris control circuit,which controls an iris drive circuit 27 based on an image signalsupplied from the sample hold circuit 24 and performs the automaticcontrol of an ig meter 28 in order to control the opening quantity ofthe diaphragm 22 so that the level of an image signal may be a fixedvalue of a predetermined level.

Reference numerals 33 and 34 denote band pass filters (BPF) performingdifferent band limiting for extracting high-frequency componentsnecessary for performing in-focus detection among the image signalsoutput from the sample hold circuit 24. The signals output from a firstband pass filter 33 (BPF1) and a second band pass filter 34 (BPF2) aregated by a gate circuit 35 with a focus gate frame signal, and the peakvalues of the gated signals are detected by a peak detection circuit 36to be held. The held peak values are input into a logic control circuit37. The input signals are called focus voltages, and focusing ispreformed based on the focus voltages.

Moreover, a reference numeral 38 denotes a focus encoder detecting amoved position of the focus lens 21A. A reference numeral 39 denotes azoom encoder detecting the focus distance of the zoom lens 21B. Areference numeral 40 denotes an iris encoder detecting an openingquantity of the diaphragm 22. The detected values of these encoders 38to 40 are supplied to the logic control circuit 37 performing systemcontrol.

The logic control circuit 37 performs in-focus detection of a subjectbased on an image signal corresponding to one in a set in-focusdetection region to perform focusing. That is, the logic control circuit37 captures the peak value information of the high-frequency componentssupplied from each of the band pass filters 33 and 34, and suppliescontrol signals of a rotation direction, a rotation speed, arotation/stop and the like of a focus motor 30 to a focus drive circuit29 in order to drive the focus lens 21A to the position where the peakvalues of the high-frequency components become a maximum. Thus, thelogic control circuit 37 controls the focus drive circuit 29.

A zooming drive circuit 31 rotates a zoom motor 32 when a zoom operationis instructed. When the zoom motor 32 rotates, the zoom lens 21B moves,and the zoom operation is performed. Also, according to the presentembodiment, a high quality movie image can be provided.

As described above, according to the image pickup device of the presentinvention, a decrease of an image signal output at the time of theincidence of a large quantity of light can be reduced and outputvariations of the pixels in the same row as that of the large lightquantity pixel, which are read at the same time as the large lightquantity pixel, can be reduced. And, by means of the above solution ofthe problem, the variation likely be caused in the image can be reduced.Thus, a high quality image can be provided.

In addition, any of the embodiments described above are only examples atthe time of implementing the present invention, and the scope of thepresent invention should not be interpreted to be limited to theembodiments. For example, the configuration of a pixel and theconfiguration of a clip circuit are not limited to those of theembodiments. Moreover, the polarities of charges are indifferent, andthe present invention can be also applied to the structure of the imagepickup device in which voltage relations are inverse. That is, thepresent invention can be implemented in various forms without departingfrom the scope and the main features thereof.

This application claims priority from Japanese Patent Application Nos.2005-169780 filed Jun. 9, 2005 and 2006-146796 filed May 26, 2006, whichare hereby incorporated by reference herein.

1. An image pickup device, comprising: a plurality of pixels, eachincluding a photoelectric conversion element converting light intocharges; a transfer gate that transfers the converted charges to afloating node; a first transistor that outputs a signal based on avoltage of the floating node to a source thereof, and a secondtransistor that resets the voltage of the floating node; a signal lineto which the first transistor is connected through the source, and towhich the first transistor outputs a signal through the source; aclipping circuit including at least a third transistor having a sourceconnected to the signal line to which the first transistor is connectedthrough the source, and being capable of suppressing a fluctuation of avoltage of the signal line at a first voltage and a second voltagedifferent from the first voltage, and a constant current source beingcapable of supplying a current from a drain to the source of the firsttransistor or a drain to the source of the third transistor, wherein areset signal reading period for reading a signal based on a voltage ofthe floating node at the time of reset of the floating node, and aphotoelectric conversion signal reading period for reading a signalbased on a voltage of the floating node at the time of transferring thecharges from the photoelectric conversion element to the floating nodeare set, the clipping circuit suppressing the fluctuation of the voltageof the signal line to the first voltage during a period of at least apart of the reset signal reading period, and suppresses the fluctuationof the voltage of the signal line to the second voltage during a periodof at least a part of the photoelectric conversion signal readingperiod, and wherein the first voltage is higher than the second voltage,and the second voltage is set so as not to turn off the constant currentsource.
 2. The image pickup device according to claim 1, wherein theclipping circuit includes a power source supplying a voltage to a drainof the third transistor, and is capable of switching the voltage of thesignal line to be at the first and second voltages by switching a gatevoltage of the third transistor.
 3. The image pickup device according toclaim 1, wherein the charges are electrons, and the first voltage ishigher than the second voltage.
 4. The image pickup device according toclaim 1, further comprising one or more A/D converters converting avoltage output from the signal line from an analog signal to a digitalsignal, wherein a difference between the first voltage and the secondvoltage is larger than a saturation range of the A/D converter.
 5. Animage pickup system, comprising: an image pickup device according toclaim 1; and a lens for focusing an optical image on the image pickupdevice.
 6. The image pickup device according to claim 1, wherein thethird transistor is connected through the source thereof to the signalline and is connected through a drain thereof to a power source, and thethird transistor is supplied through a gate thereof with a voltage forsetting, at the first voltage, the source of the third transistor andwith a second voltage, different from the first voltage, for setting, atthe second voltage, the source of the third transistor.
 7. The imagepickup device according to claim 4, further comprising a plurality ofthe signal lines and a plurality of A/D converters, wherein each of theplurality of A/D converters is connected to each of the signal lines.